Fix

Author: phofl

pandas/core/internals/managers.py

@@ -445,11 +445,15 @@ def convert(self: T, copy: bool | None) -> T:
         elif copy is None:
             copy = True
 
+        if self.is_single_block:
+            original_blocks = [self.blocks[0]]
+        else:
+            original_blocks = [self.blocks[i] for i in self.blknos]
         mgr = self.apply(
             "convert",
             copy=copy,
             using_copy_on_write=using_copy_on_write(),
-            original_blocks=[self.blocks[i] for i in self.blknos],
+            original_blocks=original_blocks,
         )
         refs = [getattr(blk, "_ref", None) is not None for blk in mgr.blocks]
         if any(ref is not None for ref in refs):